I just got a salter multiweigh kitchen scale model 810. I attempted to calibrate it with unopened paper wrapped butter quarters (1/4 # each). When I put on one quarter the scale measures 5 oz when I put on 2 it measures 8oz, 3 = 12 oz. I want to be able to measure 2 or 3 oz acurately. What am I doing wrong? Should I return it to Amazon? Georgia Bell